This invention relates generally to microelectromechanical structures (MEMS). More particularly, it relates to MEMS elements.
MEMS free-space optical switches can be categorized into two major branches: the planar matrix (2-dimensional) approach, and the beam-steering (3-dimensional) approach. The 2-dimensional approach typically involves mirrors that move between two positions (on and off). The 3-dimensional approach requires precise xe2x80x9canalogxe2x80x9d control mirror position through a wide angle to steer the beam.
In a conventional 2-dimensional approach, the MEMS switching components, such as rotating mirrors may be formed from a substrate using standard photolithography techniques. The mirrors are typically formed in the plane of the substrate and rotate out of plane into an xe2x80x9cONxe2x80x9d or up-position to steer the beam because the light beam typically travels parallel to the substrate. Only the position accuracy at the xe2x80x9cONxe2x80x9d or up-position is critical as it determines the alignment accuracy and optical loss. In the xe2x80x9cOFFxe2x80x9d or down-position, the mirror position can be xe2x80x9ccoarselyxe2x80x9d controlled. To align the mirrors in the up-position a two-chip approach is often used. A xe2x80x9ctop chipxe2x80x9d is formed having openings with perpendicular sidewalls. The openings align with the mirrors formed on a xe2x80x9cbottom chip.xe2x80x9d A xe2x80x9ctop chipxe2x80x9d having openings with almost perfectly perpendicular sidewalls may be formed, e.g., by etching a  less than 110 greater than -silicon wafer with an anisotropic etchant. When the xe2x80x9ctopxe2x80x9d chip is properly aligned and bonded to the xe2x80x9cbottom chip,xe2x80x9d the sidewalls of the openings can serve as reference stopping planes to fix the up-position of the mirrors. In addition, the sidewalls may also serve as electrodes to hold the mirrors in the up-position electrostatically. Unfortunately, the fabrication and alignment can be complicated, which reduces the yield of useful devices and correspondingly increases their cost.
It is desirable to orient the mirror in the xe2x80x9conxe2x80x9d position as fabricated. Typically, the mirrors are formed as a layer on a wafer, parallel to the wafer surface. If the mirrors are xe2x80x9conxe2x80x9d at this position, one needs to form an out-of-plane array unless many wafers are stacked. It is very difficult to build such an array. Therefore, the solution is to build mirror plates oriented perpendicular to the wafer surface. One approach to making mirrors perpendicular to the wafer surface is to fold polysilicon mirrors out-of -plane. In this approach, the mirrors are formed in-plane and may be folded and latched out-of-plane by hand using a probe. This is extremely labor intensive and the accuracy is questionable as it relies on a mechanical clamp or latch to fix the mirror in the on position. Alternatively, a micro-actuator may be used to fold the mirrors out of plane. However, the space required for a capable actuator is often somewhat larger than the mirror. Consequently, the device density, an important factor, is severely compromised.
An article by Cornel Marxer et al., titled xe2x80x9cVertical Mirrors Fabricated by Deep Reactive Ion Etching for Fiber-Optic Switching Applicationsxe2x80x9d published in the Journal of Microelectromechanical Systems, Vol. 6, No. 3, September 1997, describes the fabrication of a vertically oriented MEMS mirror from a single crystal substrate. The electrostatically actuated MEMS mirror slides horizontally, i.e., parallel to the plane of the substrate, to implement a 2xc3x972 crossbar switch. Two pairs of optical fibers are positioned in a cross (+) shaped configuration with the MEMS mirror located at the intersection of the cross. In a xe2x80x9ccrossxe2x80x9d state, the MEMS mirror is retracted from in between the fibers. In the xe2x80x9ccrossxe2x80x9d state optical signals may traverse the space between the fibers in a straight path from one fiber to a directly opposing fiber. In an xe2x80x9cadd/dropxe2x80x9d state, the mirror is interposed between the fibers to deflect optical signals by 90 from one fiber to a perpendicularly opposing fiber. Unfortunately, the switch of Marxer et al. cannot be readily scaled up to implement switches having large planar arrays of mirrors. Specifically the actuator used to move the mirror occupies space that limits the pitch or minimum spacing between mirrors.
Thus, there is a need in the art, for an optical switch having MEMS elements that overcomes he above disadvantages and a corresponding method of making it.
The disadvantages associated with the prior art are overcome by embodiments of the present invention directed to a microelectromechanical systems (MEMS) element. According to an embodiment of the invention, the MEMS element comprises a crystalline substrate having a crystal structure characterized by two or more substrate crystal axes. A moveable element is moveably attached to the substrate. The moveable element includes a perpendicular portion oriented substantially perpendicular to a plane of the substrate. The perpendicular portion of the moveable element has a crystal structure characterized by one or more moveable element crystal axes. The crystal structure of the perpendicular portion of the moveable element is substantially the same as the crystal structure of the substrate. When the moveable element is in at least one position, two or more of the moveable element crystal axes are oriented substantially parallel to two or more corresponding substrate crystal axes. In at least one position, a part of a perpendicular portion of the moveable element projects beyond a surface of the substrate. The moveable element may be retained by a latch.
In accordance with another embodiment of the present invention, A MEMS element may comprise a substrate and a moveable element. The moveable element is moveably attached to the substrate for motion substantially constrained to a plane oriented substantially perpendicular to a plane of the substrate. The moveable element has a perpendicular portion that is formed from the material of the substrate. The perpendicular portion is formed substantially perpendicular to the substrate. In at least one position, a part of a perpendicular portion of the moveable element projects beyond a surface of the substrate.
In accordance with another embodiment of the invention, an optical switch may comprise a crystalline substrate and one or more moveable elements moveably attached to the substrate. The substrate has a crystal structure characterized by two or more substrate crystal axes. Each moveable element includes a perpendicular portion oriented substantially perpendicular to a plane of the substrate. The perpendicular portion of each moveable element has a crystal structure characterized by one or more moveable element crystal axes. The crystal structure of each perpendicular portion is substantially the same as the crystal structure of the substrate. When a given moveable element is in at least one position, two or more of the moveable element crystal axes for the given element are oriented substantially parallel to two or more corresponding substrate crystal axes. In at least one position, a part of a perpendicular portion of each moveable element projects beyond a surface of the substrate.
Another embodiment of the present invention provides a method for making a microelecromechanical systems (MEMS) element. The method comprises providing a substrate; forming one or more trenches in the substrate to define a perpendicular portion of a element; and moveably attaching the moveable element to a first surface of the substrate; removing a portion of the substrate such that at least a part of the perpendicular portion projects beyond a second surface of the substrate.
The various embodiments of the present invention provide for a MEMS elements that are robust, reliable and may be densely packed. MEMS elements according to embodiments of the present invention exhibit a simple design that does not require a lengthy fabrication process. The design assures high yield and improved device performance. Fabrication turnaround time can be also reduced to improve throughput.